Close-loop class-d audio amplifier and control method thereof

ABSTRACT

The present invention discloses a Class-D power amplifier and control method thereof. In one embodiment, the amplifier feeds back the signal at the output node to the inverting input of the comparator, and provides a high frequency triangular wave signal to the non-inverting input of the comparator. In addition, the non-inverting input of the comparator may be coupled to an offset voltage, while the inverting input of the comparator may be coupled to a fixed-frequency rectangular wave signal, a feedback signal which is derived from the output stage and an input signal. In use, the switching frequency may be at least substantially fixed, so as to reduce the influence on the system caused by electromagnetic interruption (EMI). Further, the control circuit is simple, and some devices can be integrated.

RELATED APPLICATION DATA

This application is a continuation of U.S. application Ser. No.12/169,539 filed Jul. 8, 2008, which, in turn, claims the benefit of thefiling date of CN application Serial No. 200710140141.2 filed on Aug.16, 2007 and incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to a Class-D power amplifier,and particularly to a close-loop fixed-frequency Class-D power amplifierand control method thereof.

DESCRIPTION OF RELATED ART

There are many different kinds of power amplifiers, such as Class-A,Class-B, Class-AB, Class-D, etc. The Class-D power amplifier isdifferent from other amplifiers for it is a switch-mode or pulse widthmodulation (PWM) power amplifier. In such Class-D power amplifier,devices are either absolutely on, or absolutely off, which highlyreduces the power loss of the output devices. An audio signal is used tomodulate the PWM carrier signal which drives the output power stage toget a high-frequency PWM rectangular wave, and then the amplifieroutputs the audio signal to the load through a low-pass filter. Atpresent, much attention is paid to improve the power density and reducethe cost when the, Class-D power amplifier is designed. From the view ofthe circuit structure, the Class-D audio amplifier can be seen as aconventional inverter which generates an amplified audio signal from aDC power supply input according to a reference audio signal. Therefore,all conventional close-loop feedback control methods can be used inClass-D audio amplifiers such as instantaneous voltage mode feedbackcontrol or voltage&current double feedback loops control, etc. A typicaldevice used in these control methods is an error amplifier. So thecircuit structures are not only complicated but also high cost. Theanalog adaptive modulation (AAM) technology which is owned by MonolithicPower Systems® (MPS®), Inc. is a relatively simple method to realize theclose-loop control of the Class-D power amplifier.

As shown in FIG. 1, resistors R1 and R2 form a voltage divider in theAAM structure. The voltage divider provides the non-inverting input ofthe comparator Pin with an offset which equals to ½ Vcc. The comparatorhas an internal hysteresis loop. The comparator compares the inputsignal at node B with ½ Vcc±dV, wherein dV represents the hysteresisvoltage of the comparator. The output PWM wave of the comparatorcontrols the transisitors M1 and M2 to be turned on alternately throughthe drive circuit. The source of the transistor M1 is coupled to theoutput node C, while the source of the transistor M2 is grounded. Thedrain of the transistor M1 is couple to the supply Vcc, while the drainof transistor M2 is coupled to the output node C. The transistors M1 andM2 act as switches, which constitute a part of the output stage of theClass-D power amplifier circuit, so as to generate a rectangular wave atthe output node C when the output stage is used in switch mode. The SWsignal at the output node C is restored to an amplified audio signalthrough a filter circuit composed of the inductor L and capacitor C1,and the blocking capacitor C2, and then delivered to the load (forexample, a loudspeaker). Meanwhile, the SW signal charges/discharges thecapacitor Cint through the resistor Rf to realize the adaptive control.

As shown in FIG. 2, when the output PWM rectangular wave at the outputnode C is high, the capacitor Cint is charged and the voltage at Nin(node B) increases until it reaches the upper limit of the hysteresisloop. Then the high side transistor M1 is turned off and the low sidetransistor M2 is turned on, which induces the output PWM rectangularwave to be low. When the output PWM rectangular wave at the output nodeC is low, the Cint is discharged until the voltage at Nin decreases tolower limit of the hysteresis loop. Then the low side transistor M2 isturned off and the high side transistor M1 is turned on, which inducesthe output PWM rectangular wave to be high. Such process is repeated togenerate the high-frequency PWM rectangular wave at node C which will befiltered by the filter to get an amplified output audio signal. Thefeedback circuit feeds back the signal at the node C to the invertinginput of the comparator to control the output audio signal to follow theinput audio signal, and realize a certain gain amplification. The AAMtechnology allows a flexible gain set and can achieve good audioperformance both in single ended (SE) and bridge tied load (BTL)structure. However, the switching frequency varies heavily during theoperation and some electromagenetic interruption (EMI) problems mayoccur because the wide frequency spectrum may drop into the audio band(FM/AM) and decrease the sensitivity of FM/AM or disturb the videosignal sometimes, which restricts the use of this techonology onoccasion of vehicle electronics, audio broadcast and so on.

Thus, it would be advantageous to provide a system and method thatovercome these and other drawbacks of the prior art. For example, itwould be advantageous to provide a fixed-frequency Class-D poweramplifier and method thereof for reducing the influence on the systemcaused by EMI.

SUMMARY OF INVENTION

The present invention provides a method for close-loop control in aClass-D power amplifier which can keep a fixed-frequency to avoid theband of some important signals. The structure of the control circuit issimple, and some devices can be integrated.

In accordance with an embodiment, a close-loop class-D power amplifieris provided, comprising: an input stage for receiving an input signal,said input stage comprises a comparator and a triangular wave generatoror a rectangular wave generator, said comparator receives said inputsignal and the high-frequency triangular wave generated by saidtriangular wave generator or the high-frequency rectangular wavegenerated by said rectangular wave generator, and then outputs a firstsignal; an output stage coupled to said input stage, responds to saidfirst signal to generate a second signal; a filter coupled to the outputnode of said output stage, for filtering said second signal to get anoutput signal; and a feedback circuit, coupled between the output nodeof said output stage and the input node of said input stage, shapes saidsecond signal to get a feedback signal which is negatively feedback tosaid input stage, so as to subtract said feedback signal from said inputsignal.

In accordance with the power amplifier described above, saidhigh-frequency triangular wave may be applied to the non-inverting inputof said comparator, while said input signal and said feedback signal maybe applied to the inverting input of said comparator, where saidcomparator may compare said high-frequency triangular wave with a signalwhich is gained by subtracting said feedback signal from said inputsignal, so as to get said first signal.

In accordance with the power amplifier described above, the DC offsetvoltage may be applied to the non-inverting input of said comparator,said input signal, said feedback signal and said high-frequencyrectangular wave may be applied to the inverting input of saidcomparator, where said comparator may compare said DC offset voltagewith a signal which is gained by subtracting said feedback signal fromsaid input signal and then adding it to said high-frequency rectangularwave, so as to get said first signal.

In accordance with the power amplifier described above, said feedbackcircuit may comprise a capacitor, the first terminal of said capacitoroptionally coupled to said input signal, the inverting input of saidcomparator and said second signal, while the second terminal of saidcapacitor may be coupled to ground or the DC offset voltage.

In accordance with the power amplifier described above, said feedbackcircuit may further comprise a feedback resistor coupled between thefirst terminal of said capacitor and said second signal.

In accordance with the power amplifier described above, thecharge/discharge effect on said capacitor of said high-frequencytriangular wave or said high-frequency rectangular wave may be strongerthan that of said second signal.

In accordance with the power amplifier described above, said input stagemay comprise said comparator; and said output stage may comprise: adrive circuit coupled to said comparator; a half-bridge switch circuitcoupled to said drive circuit, which may be alternately turned onaccording to the drive signals generated by said drive circuit, togenerate one said second signal accordingly.

In accordance with the power amplifier described above, said input stagemay comprise said comparator; and said output stage may comprise: adrive circuit coupled to said comparator; two half-bridge switchcircuits coupled to said drive circuit, which may be alternately turnedon according to the drive signals generated by said drive circuit, togenerate two said second signals accordingly, and feed back onethereinto.

In accordance with the power amplifier described above, said input stagemay comprise two said comparators; and said output stage may comprise: adrive circuit, coupled to said comparators separately; two half-bridgeswitch circuits coupled to said drive circuit separately, which may bealternately turned on according to the drive signals generated by saiddrive circuit, to generate two said second signals accordingly, and feedback signals to corresponding comparator separately.

In accordance with another embodiment, a control method for a close-loopclass-D power amplifier is provided, comprising receiving an inputsignal and a high-frequency triangular wave generated by a triangularwave generator or a high-frequency rectangular wave generated by arectangular wave generator, and outputting a first signal; responding tosaid first signal to generate a second signal; filtering said secondsignal to get an output signal; and shaping said second signal to get afeedback signal, and feeding back said feedback signal to the inputterminal which receives said input signal, so as to subtract saidfeedback signal from said input signal.

In accordance with the control method described above, said first signalis responded in order to generate two of said second signals, at leastone of the two said second signals are shaped, and the shaped signalsare fed back to said input terminal.

The class-D power amplifier may use a comparator with a hysteresis loopwhich is small enough to be neglected instead of the hysteresiscomparator used in the AAM structure. The low frequency part of thesignal output from the power stage may be counteracted with the inputaudio signal at the input terminal of the comparator, while the highfrequency part may be sent to the comparator, so as to get a modulatedoutput audio signal at the output terminal.

The non-inverting input of the comparator may be coupled to afixed-frequency triangular wave signal, while the inverting input of thecomparator may be coupled to the feedback signal from the output stageand an input signal.

As another option, the non-inverting input of the comparator may becoupled to a DC offset voltage Vdd, while the inverting input of thecomparator may receive a fixed-frequency rectangular wave signal, thefeedback signal from the output stage and an input signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be further understood with reference to thefollowing detailed description and the appended drawings, wherein likeelements are provided with like reference signs.

FIG. 1 illustrates a drive circuit of the prior art AAM Class-D poweramplifier;

FIG. 2 is the operation waveform of the prior art AAM Class-D poweramplifier;

FIG. 3 illustrates a schematic circuit used in a SE Class-D poweramplifier, in accordance with one embodiment;

FIG. 4 illustrates the waveforms of the input audio signal and theoutput audio signal of the circuit shown in FIG. 3, in accordance withanother embodiment;

FIG. 5 illustrates the waveforms of the SW signal, the signal at thenon-inverting input Pin and the signal at the inverting input Nin of thecircuit shown in FIG. 3, in accordance with another embodiment;

FIG. 6 illustrates a schematic circuit used in a SE Class-D poweramplifier, in accordance with still another embodiment;

FIG. 7 illustrates the waveforms of the input audio signal and theoutput audio signal of the circuit shown in FIG. 6, in accordance withanother embodiment;

FIG. 8 illustrates the waveforms of the rectangular wave, the SW signal,the signal at the non-inverting input Pin and the signal at theinverting input Nin of the circuit shown in FIG. 6, in accordance withanother embodiment;

FIG. 9 illustrates a schematic circuit used in a BTL Class-D poweramplifier, in accordance with still another embodiment;

FIG. 10( a) and FIG. 10( b) illustrate part of the waveforms of thecircuit shown in FIG. 9, in accordance with another embodiment;

FIG. 11 illustrates a schematic circuit used in a BTL Class-D poweramplifier, in accordance with another embodiment;

FIG. 12( a) and FIG. 12( b) illustrate part of the waveforms of thecircuit shown in FIG. 11, in accordance with another embodiment.

FIG. 13 is the flow chart of a control method for a close-loop Class-Dpower amplifier, in accordance with another embodiment.

DETAILED DESCRIPTION

FIG. 3 illustrates a schematic circuit in accordance with oneembodiment, comprising a Class-D amplifier circuit and a load. A properhigh-frequency (about hundreds of KHz) triangular wave with ½ Vcc offsetis applied to the non-inverting input of the comparator Pin (A) whoseinverting input is Nin (B) coupled to ground through the capacitor Cint.The input audio signal charges/discharges the capacitor Cint through theresistors Ri and the capacitor Cin. The output PWM wave of thecomparator controls the transistors M1 and M2 to be turned onalternatively through the drive circuit, wherein the source of thetransistor M1 is connected to the output node C while the source of thetransistor M2 is grounded. The drain of the transistor M1 is connectedto power supply Vcc, while the drain of the transistor M2 is connectedto the output node C. The transistors M1 and M2 act as switches thatform a part of the output stage of the Class-D amplifier circuit togenerate a rectangular wave output at the output node C when the outputstage is used in switch mode. SW signal at the output node C is restoredto an amplified audio signal through a filter circuit comprising theinductor L and capacitor C1, and the blocking capacitor C2, and thendelivered to the load (for example, a loudspeaker). Meanwhile, the SWsignal charges/discharges the capacitor Cint through the resistor Rf.The charge/discharge effects produced by the audio signal and the SWsignal at the output node C may be exactly counteracted. Thus, theinverting input of the comparator Nin may keep following the voltage ofthe non-inverting input of the comparator Pin under the function of bothVsw_low and the audio input signal. When the output of the comparator ishigh, the transistor M1 is turned on and the transistor M2 is turnedoff. The voltage V_(Nin) at the inverting input of the comparator Nin iscompared with a sum of the voltage at the non-inverting input of thecomparator Pin and the hysteretic voltage dV, Vpin+dV (wherein dVrepresents hysteretic voltage of the comparator). The SW signal is highat this time, which after feedback causes the voltage at Nin to raisetill it becomes larger than the voltage Vpin+dV. Then the output of thecomparator becomes low, the transistor M1 is turned off and thetransistor M2 is turned on. The voltage at the inverting input of thecomparator Nin is compared with the voltage Vpin−dV at this time. Thefeedback of the low SW signal causes the voltage at Nin to drop till itbecomes less than the voltage Vpin−dV. The output of the comparatorbecomes high, causing the SW signal to be high, circularly (e.g. asshown in FIG. 5). Therefore, to realize fixed-frequency feedback controland ensure the system working steadily, the raising and dropping ratesof the voltage at Nin may optionally be less than the changing slope ofthe given triangular wave at Pin. So the changing slope of thetriangular wave may be taken into consideration when the capacitor Cintand the feedback resistor Rf are designed. The gate drive signals of thetransistors M1 and M2 can be gained from the high-frequency part of theSW signal under the modulation of the high-frequency triangular carrierwave of non-inverting input Pin. The gain of the amplifier is confirmedby the ratio of resistors Rf and Ri.

Optional embodiments of key operation waveforms of the circuit in FIG. 3are illustrated in FIG. 4 and FIG. 5. Switch control method inaccordance with another embodiment is apparent in FIG. 5. When the inputaudio signal changes, the SW signal can be adaptively modulated by thissystem to let the voltage at the inverting input Nin always follow thenon-inverting input Pin so as to control the output. Of course, theswitching frequency is not unchangeable but has a minor change at therange of hundreds of HZ. This change is caused by the small audio sinesignal at the inverting input of the comparator which also cancharge/discharge the capacitor Cint. However, the change like this isvery small relative to the switching frequency, so this control methodcan be seemed as a fixed-frequency control all the same.

FIG. 6 illustrates a schematic circuit in accordance with anotherembodiment. Its basic configuration is similar to FIG. 1 except that thetriangular wave applied to the non-inverting input Pin in FIG. 1 isreplaced by a rectangular wave with ½ Vcc offset applied to theinverting input Nin via the resistor Rs while the non-inverting input ofthe comparator Pin is directly coupled to ½ Vcc DC offset. Thecharge/discharge effect on the capacitor Cint of the rectangular wave issimilar to the effect of the triangular wave directly applied to thenon-inverting input Pin of the comparator. In former embodiment, theslope rate of the given triangular wave may need to be always largerthan that of the voltage at the inverting input Nin, i.e. thehigh-frequency charge/discharge ripple at the integral capacitor Cint.The charge/discharge effect on the capacitor Cint of the triangular wavemay also need to be stronger than the charge/discharge effect of thefeedback signal SW. Likewise, in this embodiment, the charge/dischargeeffect on the capacitor Cint of the given rectangular wave may need tobe stronger than the effect of the feedback signal SW.

Optional embodiments of key operation waveforms of the circuit in FIG. 6are illustrated in FIG. 7 and FIG. 8. Referring to FIG. 8, a periodcycle can be divided into 5 phases:

Phase 1 (t0-t1): At t=t0, the rectangular wave becomes low. The SWsignal and the rectangular wave signal discharge the capacitor Cint atthe same time. The voltage Vcint of the capacitor Cint keeps falling.

Phase 2 (t1-t2): At t=t1, Vcint<½ Vcc, the output of the comparator isreversed, and the SW signal becomes high. The SW signal charges Cintwhile the rectangular wave keeps discharging Cint at the same time.Since the discharge effect is stronger, Vcint keeps falling at a slowrate.

Phase 3 (t2-t3): At t=t2, the rectangular wave becomes high. The SWsignal and the rectangular wave charge the Cint at the same time. Vcintraises.

Phase 4 (t3-t4): At t=t3, Vcint>½ Vcc, the output of the comparator isreversed again, and the SW signal becomes low. The SW signal dischargesCint while the rectangular wave keeps charging Cint at the same time.Since the charge effect is stronger, Vcint keeps raising at a slow rate.

Phase 5 (t4-t5): At t=t4, the rectangular wave becomes low. The SWsignal and the rectangular wave discharge the Cint at the same time.Vcint falls.

As mentioned before, to realize the proposed fixed-frequency feedbackcontrol of this embodiment, the voltage at Nin may be required to keepfalling when the SW signal becomes high in Phase 2 and keep raising inPhase 4. The following formula may need to be fulfilled while thefeedback resistor Rf, the voltage Vrectangular of the rectangular wavewith ½ Vcc offset, the SW signal Vsw and the resistor Rs are designed:

${\frac{V_{rectangular} - V_{Cint}}{R_{s}}} > {\frac{V_{sw} - V_{Cint}}{R_{f}}}$

Accordingly, the charge/discharge effect on the capacitor Cint of thegiven rectangular wave may be required to be stronger than the effect ofthe feedback signal SW in this embodiment. Since the charge/dischargeeffect on the capacitor of the rectangular wave is greatly stronger thanthe effect of feedback signal SW, although there is a change withhundreds of HZ, the frequency of the SW signal which is decided by thefrequency of the rectangular wave is fixed as a whole.

Similar to the SE Class-D power amplifier mentioned before, the presentinvention also can be used in BTL power amplifier. Harmonic distortionand DC offset can be eliminated by the inherence differential outputstructure of the bridge type topology. FIG. 9 illustrates a schematiccircuit in accordance with another embodiment. The H-bridge comprises 2half-bridge switching circuits which are powered by single power supplyVcc generally. For given Vcc, the max amplitude of the output signal inH-bridge circuit is 2 timers larger than which in single ended manner,while the output power is 4 timers larger. Only one comparator is used,whose output controls the transistors S1,S2,S3 and S4 to be turned onalternately throuth the drive circuit so as to get two phase oppositesignals SW1 and SW2 which are delivered to the load through the filterL1, C1 and L2, C2. Only one of the SW1 and SW2 may need to be fed backand used in feedback control loop. In FIG. 9 SW2 is used as a feedbacksignal. FIG. 10( a) illustrates optional waveforms of audio input andaudio output of the circuit shown in FIG. 9, while FIG. 10( b)illustrates optional partly magnified operation waveforms in which theSW1 signal can be gained through the phase-reversal of the SW2 signal.

FIG. 11 illustrates a schematic circuit in accordance with anotherembodiment in BTL amplifier systems. In this embodiment, eachhalf-bridge has its own special comparator to control two drive circuitsseparately. The switching frequency of each bridge is the same sincethey are set by the same external triangular wave at the Node A. Thestructure of the control circuit of each bridge is similar to that inthe SE amplifier except the absence of the blocking capacitor (referringto the capacitor C2 in FIG. 3 and FIG. 6), and the gain may also becalculated by Rf/Ri. The input audio signal is a differential signalwhich is phase opposite, applied at the inverting inputs B1 and B2 ofthe two comparators and compared with the triangular wave. FIG. 12( a)illustrates an embodiment of the waveforms of the audio input and audiooutput of the circuit shown in FIG. 11, while FIG. 12( b) illustrates anembodiment of the partly magnified operation waveforms.

The drive circuits in FIG. 3, FIG. 6, FIG. 9 and FIG. 11 can beimplemented by gate drive circuit or other circuits which can achievethe same function. as an option. In addition, the number of the drivecircuits in FIG. 3, FIG. 6, FIG. 9 and FIG. 11 is only schematicallyshown by way of example, and thus only needs to fulfill that the drivecircuits can be controlled by respective comparator and drive respectivetransistor, rather than be the same with the number of the blocksrepresentative of the drive circuits in the Figures mentioned above.

Referring to FIG. 13, a control method for a close-loop Class-D poweramplifier in accordance with one embodiment comprises the followingoperations:

Receive an input signal and a high-frequency triangular wave generatedby a triangular wave generator or a high-frequency rectangular wavegenerated by a rectangular wave generator, and output a first signal;

Respond to said first signal and generate a second signal, e.g. the SWsignal;

Filter the SW signal to get an output signal; and

Shape the SW signal to get a feedback signal, and feed back saidfeedback signal to the input terminal which also receives said inputsignal, so as to subtract the feedback signal from said input signal.Compared to the prior art AAM scheme, the present embodiment may onlyneed to add a DC power supply with a ½ Vcc offset and a propertriangular wave or rectangular wave. After the integration of theseparts, the close-loop fixed-frequency control of the Class-D poweramplifier can be simply achieved.

The above detailed description of embodiments of the present inventionis not intended to be exhaustive or to limit the invention to theprecise form disclosed above. While specific embodiments of, andexamples for, the invention are described above for illustrativepurposes, various equivalent modifications are possible within the scopeof the invention, as those skilled in the relevant art will recognize.

The terminology used in the Detailed Description is intended to beinterpreted in its broadest reasonable manner, even though it is beingused in conjunction with a detailed description of certain specificembodiments of the invention. Certain terms may even be emphasized;however, any terminology intended to be interpreted in any restrictedmanner will be overtly and specifically defined as such in this DetailedDescription section. In general, the terms used in the following claimsshould not be construed to limit the invention to the specificembodiments disclosed in the specification, unless the above DetailedDescription section explicitly defines such terms. Accordingly, theactual scope of the invention encompasses not only the disclosedembodiments, but also all equivalent ways of practicing or implementingthe invention under the claims.

1.-20. (canceled)
 21. A close-loop class-D power amplifier, comprising:an input stage that receives an input signal, a feedback signal and acarrier signal, operable to provide a comparison signal in response tothe input signal, the feedback signal and the carrier signal; an outputstage that receives the comparison signal, and based thereupon, providesan amplified signal; a feedback circuit that receives the amplifiedsignal, and based thereupon, provides the feedback signal to the inputstage; wherein the output stage comprises at least two switches, theswitches are turned on alternately at any time when the input signal,the feedback signal and the carrier signal meet a predeterminedcondition.
 22. The class-D power amplifier of claim 21, wherein theinput stage comprises a comparator and a capacitor coupled to thecomparator.
 23. The class-D power amplifier of claim 22, furthercomprising a triangular wave generator for generating a triangular wavesignal as the carrier signal.
 24. The class-D power amplifier of claim23, wherein the capacitor, the input signal and the feedback signal arecoupled to the inverting input of the comparator, the triangular wavesignal is coupled to the non-inverting input of the comparator.
 25. Theclass-D power amplifier of claim 23, wherein the predetermined conditionis that the triangular wave signal is higher than the voltage across thecapacitor.
 26. The class-D power amplifier of claim 22, furthercomprising a rectangular wave generator for generating a rectangularwave signal as the carrier signal.
 27. The class-D power amplifier ofclaim 26, wherein the capacitor, the input signal, the feedback signal,and the rectangular wave signal are coupled to the inverting input ofthe comparator, and a DC offset is coupled to the non-inverting input ofthe comparator.
 28. The class-D power amplifier of claim 27, wherein thepredetermined condition is that the voltage across the capacitor islower than the DC offset.
 29. The class-D power amplifier of claim 21,further comprising a filter that receives the amplified signal, andbased thereupon, provides an output signal.
 30. The class-D poweramplifier of claim 29, wherein the output signal is used to drive aspeaker.
 31. The class-D power amplifier of claim 21, wherein the outputstage comprises: a drive circuit that receives the comparison signal,and based thereupon, provides a drive signal; a half-bridge switchcircuit coupled to the drive circuit, wherein the half-bridge switchcircuit is turned on alternately in response to the drive signal, so asto provide the amplified signal.
 32. A method for a close-loop Class-Dpower amplifier, comprising: receiving an input signal, a feedbacksignal, and a carrier signal, and providing a comparison signalthereupon; providing an amplified signal in response to the comparisonsignal; providing the feedback signal in response to the amplifiedsignal; alternately turning on switches in an output stage in responseto the comparison signal at any time when the input signal, the feedbacksignal and the carrier signal meet a predetermined condition, so as toprovide the amplified signal.
 33. The method of claim 32, furthercomprising filtering the amplified signal to get an output signal. 34.The method of claim 33, wherein the output signal is used to drive aspeaker.
 35. The method of claim 32, wherein the carrier signal is atriangular wave signal.
 36. The method of claim 35, wherein a capacitoris charged by the difference between the input signal and the feedbacksignal; and the predetermined condition is that the triangular wavesignal is higher than the voltage across the capacitor.
 37. The methodof claim 32, wherein the carrier signal is a rectangular wave signal.38. The method of claim 37, wherein a capacitor is charged by therectangular wave signal added by the difference between the input signaland the feedback signal; and the predetermined condition is that thevoltage across the capacitor is lower than a DC offset.
 39. The methodof claim 32, wherein the output stage comprises: a drive circuit forproviding a drive signal in response to the comparison signal; ahalf-bridge switch circuit coupled to the drive circuit, wherein thehalf-bridge switch circuit is turned on alternately in response to thedrive signals, so as to provide the amplified signal.